In lighting systems, having become an indispensable part in modern life, a variety of light sources are used for the enhancement of brightness in specific sites. Further, as technology advances, certain requirements for brightness, color temperature, light distribution pattern and etc., needed in each site, especially in sites involving personal safety are provided. Thus, there are strict requirements for street lights, vehicle lamps and so on.
A street light, such as “Street light led” disclosed in U. S. patent US20120106156, comprises a pole, a solid state light emitting device provided on the pole, and an optical element provided on the pole. A light is emitted from the solid state light emitting device. Moreover, a light distribution pattern complying with requirements may be produced from the light by the optical element and then emitted.
Only one single color temperature, however, is inherent in such a kind of street light. It is possibly not clear enough in general conditions, if warm color temperature is used. Nevertheless, it is clearer if cool color temperature is used, in comparison with warm color temperature, though the problem of poor visibility occurs because the cool light is apt to be reflected by water vapor in the rain or mist. Therefore, how to change color temperature of the light source according to the external environment is truly the topic with which the industry is confronted.